This invention relates generally to fiber optic acoustic sensors and particularly to fiber optic acoustic sensors for underwater use. Still more particularly, this invention relates to apparatus and methods for preventing polarization signal fading in a fiber optic interferometric sensor used as a hydrophone.
A single mode optical fiber is capable of guiding signals of two linear polarizations. In a fiber optic hydrophone comprised of fiber optic interferometric sensors, the output signal may be a function of the polarization of the optical signals guided by the optical fibers in the sensors. Generally the maximum signal is obtained if the two fibers guide signals of the same polarization state.
A major problem of fiber optic interferometric sensors is loss of signal due to signal fading caused by changes in the polarization of the waves guided by the fibers. Drift in the relative optical path length difference in the interferometer arms causes a change in the relative state of polarization of the two interferometer arms. This may also be viewed as a change in the interference phase, which changes the signal intensity. Complete polarization fading occurs when the polarizations are orthogonal. In prior art systems, polarization signal fading may degrade the signal intensity to the point that no usable information is available.
Polarization fading and strumming noise have been observed regularly in sea trials of fiber optic towed hydrophone arrays. Polarization fading in such hydrophone arrays occurs when the two fibers that comprise the hydrophone have orthogonal polarization components. Polarization fading may reduce the fringe visibility in the output of interferometric sensors to zero. All hydrophone signal information then disappears.
A fiber optic polarization controller has been used to overcome the problem of polarization signal fading. This polarization controller has a plurality of loops of the optical fiber wound on spools whose edges are mounted on a common axis. The axis lies in the plane of each coil. Adjusting the angles of the loops of optical fiber adjusts the polarization state of the optical signal guided by the fiber.
Birefringence can be induced in a single mode optical fiber by bending the fiber into a coil. Bending an optical fiber causes an increase in the material density in the place perpendicular to the plane of the coil, which increases the refractive index in that plane. Changes in the refractive index in the plane of the coil are negligible due to the opposite effect of compression on the interior and tension on the exterior part of the curvature. The stress, and therefore the change in the refractive index, is essentially constant across the fiber core. In an isotropic material the change in refractive index may be expressed as: ##EQU1## where: n is the refractive index;
s is the Poisson ratio; PA1 p.sub.11 -p.sub.12 are components of the photoelastic tensor; PA1 r is the radius of the fiber; and PA1 R is the radius of curvature of the bend.
For silica s=0.16, p.sub.11 =0.121, p.sub.12 =0.270 and n=1.46 at wavelength .lambda.=633 nm. Using these values, Equation (1) reduces to: ##EQU2## where a is a constant and is equal to 0.133.
Using Equation (2) one may calculate the radius of curvature R of a single fiber loop for any selected phase delay. For example, the requirement for a quarter wave plate is 90.degree. phase difference, and the radius R may be calculated as follows: ##EQU3## where .DELTA..beta. is the difference of propagation constants for the two possible linear polarizations and % is the wavelength in air.
The radius of curvature R may then be written as: ##EQU4##
For a wavelength of 1300 nm the value of R is about 10 mm. Rotating one of the coils simply rotates the fast and slow axes of the loop with respect to the input electric field. If a conversion of an arbitrary input polarization state to an arbitrary output polarization is desired, then combination and proper orientation of two loops of the polarization controller provides the desired transformation.
The prior art has the disadvantage of requiring a manual adjustment every few minutes. This is not practical for a multi-sensor array that may contain seven or more sensors because such an array would be cumbersome and bulky.